The present invention relates to a vertical deflection circuit which is used for a braun tube or a cathode-ray tube (CRT) for a television receiver, a computer terminal apparatus, and/or an office automation apparatus.
A cathode-ray tube for a display screen must have a horizontal deflection circuit and a vertical deflection circuit for scanning a screen by electron beam. Among them, the present invention relates to a vertical deflection circuit with improved linearity.
FIG. 3 shows a circuit diagram of a prior vertical deflection circuit. In the figure, the numeral 1 is a vertical oscillation stage, 2 is an amplifier, 3 is a vertical deflection coil which is mounted on a neck of a CRT (not shown), 4 is a capacitor for preventing DC (direct current) current, 5 is a feedback resistor, 6 is a variable resistor, 7 is a fixed resistor, 8 is a feedback capacitor, 9 is a resistor which functions to charge the feedback capacitor with constant current.
In FIG. 3, a feedback circuit is provided so that the potential at the point A, which is one of the inputs of the amplifier 2, is the same potential as that at point B, which is the other input of the amplifier 2. Accordingly, the current I in the resistor 9 is shown by the following equation. EQU I=(V.sub.cc -E.sub.1)/R (1)
In the above equation, V.sub.cc is the voltage at the DC power source terminal 12, E.sub.1 is the voltage of the DC power source 11, R is the resistance of the resistor 9. The current I is supposed to be constant current.
The capacitor 8 is charged by the constant current I through the resistor 9 during a scanning period (when the oscillation output transistor 10 is in an OFF state). Therefore, the voltage v at one end A of the resistor 9 is shown by the following equation. EQU v=I.times.t (0.ltoreq.t.ltoreq.T)
where T is the scanning period of vertical deflection, and t is the time. It should be noted that the waveform of the voltage v is a saw-tooth wave.
The saw-tooth wave voltage v is amplified by the amplifier 2, which applies the linear deflection current to the vertical deflection coil 3. Although a current amplifier is provided between the output of the amplifier 2 and the deflection coil 3, the current amplifier is not shown in the drawing for the sake of simplicity, as it is of no concern to the present invention.
However, a deflection circuit of FIG. 3 has the disadvantage as explained below.
It should be appreciated that a screen of a CRT display tube is generally curved, but the curve is not an exact arc of a circle. Generally, the curved screen is flat at the upper portion and the lower portion when compared with a complete arc screen. Therefore, the deflection length of an electron beam is larger at the upper portion and the lower portion when compared with the deflection length at the central portion of the screen. This causes the distortion of a displayed picture. Accordingly, when a circle is displayed, it is deformed when it is positioned at the peripheral portion of the screen, although it is displayed accurately if it is displayed at the centeral portion of the screen. In FIG. 4, the circle at the center of the screen is displayed accurately, however, the four circles at the peripheral portions of the screen are deformed, and the vertical length is elongated as compared with the horizontal length.
In order to solve the above problem, some improvements have been proposed. The Japanese patent publication No. 32934/76 which claims the convention priority of GB No. 11212/69 dated Mar. 3, 1969, and Japanese laid open patent publication No. 145165/81 are some of them. In those prior arts, a saw-tooth wave is modified by an integration circuit provided between the junction point of the deflection coil 3 and the capacitor 4, and the point A. The integration circuit provides an S-shaped compensation to the saw-tooth wave so that the amplitude of the saw-tooth wave at the beginning point and the end point in each vertical scanning period is a little decreased.
Although the prior S-shaped compensation can get rid of distortion at the peripheral portions of the screen, it has still the disadvantages as explained below.
Firstly, the upper portion of the screen can not be compensated independently from the lower portion of the screen. This means that the amplitude at the beginning of a saw-tooth wave can not be independently adjusted from the amplitude at the end of the saw-tooth wave, by an integration circuit. Therefore, if a screen is not exactly symmetrical electrically by some manufacturing error, the distortion is not compensated at all.
Secondly, the time constant of an integration circuit must be designed for each specific deflection circuit. However, the vertical deflection frequency distributes at least between 40 Hz and 80 Hz. For instance, the NTSC system uses 60 Hz, the PAL system 50 Hz, and a computer display system uses 80 Hz. A prior S-shaped compensation system using an integration circuit can not be used for those variety of deflection frequencies.